With regard to the first of the potential uses mentioned above in which power converters are used in electricity generation applications, it is possible to convert wind energy into electrical energy by using a wind turbine to drive the rotor of a generator, either directly or indirectly by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator (the “stator voltage”) is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level.
For optimum energy capture, the speed of rotation of the output shaft of the wind turbine will vary according to the speed of the wind driving the turbine blades. To limit the energy capture at high wind speeds, the speed of rotation of the output shaft is controlled by altering the pitch of the turbine blades. Connection of the variable voltage and frequency of the generator to the nominally fixed voltage and frequency of the supply network can be achieved by using suitably configured power converters.
A power converter in the form of a generator bridge, and typically operating as an active rectifier, is used to supply power from the generator to a dc link. The generator bridge can have any suitable topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy.
The dc output voltage of the generator bridge is fed to the dc terminals of a power converter in the form of a network bridge and typically operating as an active inverter. The principal control for the dc link voltage is achieved by controlling the generator bridge, but other methods of controlling the dc link voltage are possible.
The ac output voltage of the network bridge is filtered and supplied to the nominally fixed frequency supply network via a step-up transformer. Protective switchgear can be included to provide a reliable connection to the supply network and to isolate the generator and converter system from the supply network for various operational and non-operational requirements.
With regard to the second of the potential uses mentioned above, power converters can also be used in motoring applications. In this case, a power converter in the form of a network bridge and typically operating as an active rectifier supplies power to a dc link. The dc output voltage of the network bridge is fed to the dc terminals of a power converter in the form of a machine bridge which typically operates as an active inverter. The ac output voltage of the machine bridge is finally supplied to a variable speed ac motor.
In some applications employing three-phase power supplies, such as those outlined above, an element of redundancy is required to ensure that a reliable source of power can be provided. The required redundancy can be achieved by connecting several power converters in parallel. It can also be desirable to connect several power converters in parallel in applications where high power and/or high performance is/are required.
FIG. 1 is a schematic drawing showing part of a power conversion system in which two power converters 10, 12 are connected in parallel. The ac terminals of both power converters 10, 12 are connected to an ac electrical machine 14, which may be a generator or a motor, whilst the dc terminals of both power converters 10, 12 are connected to a dc link 16.
In the event that there is any desynchronisation between the PWM command signals of the PWM strategies of the parallel-connected power converters 10, 12, it is possible for a circulating current (denoted icirc) to flow around the loop formed by the power converters 10, 12. The circulating current is not limited to the three-phase supply frequency and can possess both ac and dc components. The presence of a circulating current is undesirable because it does not process useful power and places extra stress on the power converters 10, 12. The circulating current can, in fact, be destructive if it is allowed to become excessively large.
A number of techniques for reducing or eliminating circulating current in parallel-connected power converters have been proposed, but all of the known techniques have certain shortcomings.
In one known technique, illustrated in FIG. 2, an isolation transformer 18 is installed in the three-phase supply path of all but one of the power converters 10, 12. The isolation transformer 18 electrically separates the input circuits, whilst allowing the transmission of ac signal/power, and thus prevents any unwanted circulating current from flowing between the parallel-connected power converters 10, 12. Isolation transformers are, however, very expensive and take up a large amount of space and because of this are far from being an ideal solution.
An alternative technique involves the use of a common modulator to generate a common PWM modulating sinusoidal voltage signal for the PWM strategies of all of the power converters. The common modulating sinusoidal voltage signal is fed to each of the plurality of parallel-connected power converters to maintain the synchronisation between the power converters. In one implementation of this technique, described in U.S. Pat. No. 5,657,217, the on/off commands for the switching devices of each of a plurality of parallel-connected power converters are provided by a spatial voltage vector calculator, which selects multiple spatial voltage vectors, and a vector permutation device, which determines two sets of the order of generation of the selected multiple spatial voltage vectors.
The use of a common modulator does not, however, provide any system redundancy and in the event of failure of the common modulator, all of the parallel-connected power converters will cease functioning.
In another technique, a synchronising signal source can be used to generate a square wave that is coupled to each power converter. The modulator of each power converter uses the synchronising signal to synchronise the phase angle of the output signal with the output signals of the other power converters. Examples of different implementations of this technique are described in U.S. Pat. Nos. 5,436,823 and 4,802,079. The implementation described in U.S. Pat. No. 4,802,079 suffers from the particular drawback that it can only be used with two parallel-connected power converters.
The synchronising signal technique, like the common modulator technique outlined above, does not provide any system redundancy and, in the event of failure of the device generating the synchronising signal, it will not be possible to maintain the synchronisation of the parallel-connected power converters.
Another known technique is the so-called ‘master/slave’ technique. In this technique, one of a plurality of parallel-connected power converters is designated as the ‘master’ power converter and this master power converter sends a synchronising signal to all of the other power converters which are designated as ‘slave’ power converters. The phase angle of the triangular voltage carrier signal or the modulating sinusoidal voltage signal of the PWM strategy of each of the slave power converters is modified to achieve synchronisation of the power converters. If interchange capability is provided so that any of the power converters can assume the role of the master power converter or slave power converters, as described in U.S. Pat. No. 5,757,634, the system can continue to operate in synchronisation in the event of failure of one or more of the power converters. However, a drawback of the ‘master/slave’ technique is that a communication link is needed between the power converters, thus increasing the complexity of the power conversion system.
U.S. Pat. No. 7,327,588 B2 describes a method for synchronising a plurality of parallel-connected power converters operating as inverter units. Each inverter unit is provided with an inverter-specific modulator, thus avoiding the need for a communication link between the inverter units. Synchronisation is achieved at each modulator by stepping up or down the frequency of a triangular voltage carrier wave based on the circulating current measured at the peak of the triangular carrier wave. Although synchronisation can be achieved using this method, the frequency changes of the triangular carrier wave result in a change in the switching period and this leads to unwanted effects such as a variable set of harmonic components in the output voltage.
There is, therefore, a need for an improved method for controlling the synchronisation of a plurality of parallel-connected power converters which avoids the drawbacks associated with known techniques. In particular, there is a need for an improved method which avoids the need for both a communication link between power converters and a change in the switching period of the PWM voltage carrier signals of individual power converters.